1,3,5-triamino-2,4,6-trinitrobenzene is a powerful explosive of particularly high insensitivity to shocks, vibrations, impacts and fire making it an explosive of choice for the weapons industry.
This explosive is obtained in particular by nitration of 3,5-dichloroanisole followed by amination of the resulting trinitrated compound. One method allowing these reactions of nitration and amination to be carried out is described in U.S. Pat. No. 4,952,733 (hereinafter reference [1]).
3,5-dichloroanisole is commercially available but is relatively costly to purchase, hence the advantage for explosive manufacturers to be able themselves to conduct the synthesis thereof.
The literature regarding the synthesis of 3,5-dichloroanisole is extremely sparse and in practice is limited to an article published in 1983 by L. Testaferri et al. in Tetrahedron, 39(1), 193-197 (hereinafter reference [2]) and to the Japanese patent application published under number 53-31631 (hereinafter reference [3]).
Reference [2] teaches that it is possible to obtain mono-, di- and trichloroanisoles by nucleophilic substitution of the corresponding di-, tri- and tetra-chlorobenzenes by methanolate ions on the express condition that this substitution reaction is conducted in hexamethylphosphoramide (HMPA), also known as <<hexamethylphosphoric triamide>>.
Therefore, in this reference, 3,5-dichloroanisole is synthesized by adding sodium methanolate to a suspension, previously held under nitrogen at 120° C., of 1,3,5-trichlorobenzene in HMPA and leaving to react for one hour. It is then isolated by extraction with ether and purified by chromatography on a silica gel column using a mixture of petroleum ether and ethyl ether as eluent.
The yield of this synthesis is 78%.
It happens that HMPA is classified under the regulations of the European Union among CMR substances i.e. substances which are considered as potentially or proven to be carcinogenic, mutagenic and/or toxic for reproduction, making the use of this solvent prohibitive in a synthesis method intended to be implemented on industrial scale.
Also, the fact that the 3,5-dichloroanisole must be purified on completion of the synthesis by column chromatography in particular, means that the production of this compound is made cumbersome by increasing the number of operations to be conducted as well as the volume of consumed organic solvents, and is economically penalizing for production on an industrial scale.
Reference [3] proposes synthesizing alkoxy aromatic compounds such as 3,5-dichloroanisole, by causing the corresponding haloaromatic compounds to react with an alkoxide of an alkaline metal in the presence of N,N′-dimethylimidazolidinone.
Therefore, in this reference, 3,5-dichloroanisole is synthesized by causing 1,3,5-trichlorobenzene to react with sodium methanolate in N,N′-dimethylimidazolidinone, for 18 hours at a temperature of 100±5° C. The 3,5-dichloroanisole is then extracted from the reaction medium using ether.
N,N′-dimethylimidazolidinone, compared with HMPA, has the advantage of not being classified among CMR substances by the European Union regulation.
On the other hand, reference [3] shows that the yield of the synthesis of 3,5-dichloroanisole in this solvent is largely insufficient since it is only 47.2%.
The Inventors have set themselves the objective of providing a method which, while not using any solvent considered to have potential or proven harmful effects on the health of man or animals, allows the synthesis of 3,5-dichloroanisole at the lowest possible cost.
From this perspective, the Inventors have more particularly set themselves the goal that this synthesis method should have a high yield, should lead to obtaining sufficiently pure 3,5-dichloroanisole so that it is not necessary for it to undergo any additional purification operation after this synthesis, and should only have recourse to operations that are simple to implement.